Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) in a transmit mode (e.g., a speaker application), and/or convert received acoustic waves to electrical signals in a receive mode (e.g., a microphone application). Transducers are used in a wide variety of electronic applications. For example, transducers may be included in film bulk acoustic resonators (FBARs), surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators used in portable communication devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and the like. Generally, various types of transducers include an acoustic resonator stack, having a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane.
FIG. 1A is a cross-sectional diagram illustrating an acoustic resonator of a conventional transducer device, which has been formed according to a conventional fabrication process. Referring to FIG. 1A, acoustic resonator 130 is formed by a piezoelectric layer 134 to a first electrode layer 132 is applied to or grown on the first electrode layer 132, and a second electrode layer is applied to the piezoelectric layer 134. The first and second electrode layers 131 and 135 are formed of electrically conductive materials, such as tungsten (W), molybdenum (Mo), and the piezoelectric layer 134 is formed of a thin film of piezoelectrice material, such as zinc oxide (ZnO), aluminum nitride (AlN) or lead zirconium titanate (PZT). The arrows in the piezoelectric layer 134 indicate generally the randomly oriented growth of a ZnO thin film, for example, as discussed below.
More particularly, a ZnO thin film may be deposited with two specific crystal orientations. One crystal orientation is wurtzite (B4) structure consisting of a hexagonal crystal with alternating layers of Zn and oxygen (O) atoms, and the other crystal orientation is a zincblende (B3) structure consisting of a symmetric arrangement of Zn and O atoms. Zincblende structures grow predominantly on cubic substrates. The energetically preferred and more common structure is the wurtzite structure. Due to the nature of the Zn—O bonding in the wurtzite structure, electric field polarization is present in the ZnO crystal, which results in the piezoelectric properties of the ZnO material. To exploit this polarization and the corresponding piezoelectric effect, the ZnO should be synthesized with a specific crystal orientation, discussed below.
FIG. 1B is an expanded view of a representative ZnO crystal in a wurtzite structure. Referring to the orientation shown in FIG. 1B, the a-axis and the b-axis are in the planes of the top and bottom hexagons of the ZnO crystal, while the c-axis is parallel to the sides of the hexagons. For ZnO, the piezoelectric coefficient d33 along the c-axis is about 5.9 pm/V2. In order to take advantage of this piezoelectric coefficient, all of the ZnO crystals need to be oriented in substantially the same direction. If they are not, as shown in FIG. 1A for example, the properties of the ZnO crystals may cancel each other out or achieve a piezoelectric coefficient less than the maximum. Referring again to FIG. 1A, the arrows correspond to the c-axis directions of multiple ZnO crystals. The random mixture of c-axis orientations in the ZnO thin film prevents good piezoelectric response.